Free piston gas generator



April 25, 1961 J. G. MACDONALD FREE PISTON GAS GENERATOR Filed March 2,1959 INV ENT E] R JOHN G MAC/J ONAL D ice Patented Apr. 25,1961

2,931,242 FREE PISTON GAS GENERATOR John G. Macdonaid, P1). Box 476,Bayfieid Road, Goderich, Ontario, {Janada Filed Mar. 2, 19 59, Ser. No.796,535

2 Claims. c1. 123-46) This invention concerns improvements infree-piston machines used to produce hot gas under pressure which may beused to drive a turbine from which useful work is obtained.

This invention provides a free-piston gas generator which is positiveand easy to start, stable, efficient and reliable in operation, simpleto maintain and relatively inexpensive to manufacture. Certain featuresof-this invention, including the piston construction, synchronizing andmechanism and power take-off to operate a fuel injection pump are notshown in this specification as they form the subject of my US. PatentNo. 2,865,349, Lubricating and Sychronizing Means for Free-PistonEngines, which may be readily applied to the present invention.

As will be understood, the present engine operates on the two strokediesel cycle and comprises a power cylinder containing a pair of opposedpistons which form a part of compressor pistons operating in compressorcylinders at opposite ends of the power cylinder. The outer portion ofeach of the compressor cylinders form bounce chambers which act as aircushions to return the pistons on the inward stroke. The inner portionof the compressor cylinders. serve as air compressors to provide air forscavenging the power cylinder.

According to the invention, the engine is characterized by the fact thatall the air from the compressors is directed through the power cylinderwhere it mixes with the exhaust gases to form hot gas under relativelyhigh pressure.

Further,-according to the invention, the engine starting mechanismincludes means for suddenly releasing a given quantity of compressed airinto each of the bounce chambers and novel means are provided to insurethat the air is released into both bounce chambers simultaneously toavoid stress on the synchronizing mechanism. Novel means are alsoprovided to seal oif the bounce chambers from the starting mechanism assoon as the air has been released into said chambers and again novelmeans are provided to cushion the action of the starting mechanism toavoid damage from shock.

Another feature of this invention resides in providing means by whichthe excess air introduced for starting is released from the bouncechambers during a part of the first inward piston stroke into thescavenging airbox, the said air release means also forming a part of abounce chamber control which automatically relates the quantity of airin the bounce chambers to the prevailing air pressure in the airbox sothat as the air pressure inthe airbox is increased at higher powerlevels, a greater quantity of air is transferred to the bounce chambers,and as the pressure in the airbox falls, at lower power levels, thequantity of air in the bounce chambers is reduced by returning excessair to the airbox. Thus, the minimum pressure in t a part of thecompressor cylinders and is controlled by the motion of the compressorpistons thus requiring a minimum number of parts. The construction alsoprovides that the pressure of air in the airbox is in communication withthe bounce chambers during a substantial portion of the piston stroke sothat there will always be adequate cushioning air in the bounce chambersat all power levels.

Another important feature is the novel construction of the engine casingand attendent parts thereof which is constructed to provide perfectalignment of the opposed pistons in their respective cylinders.

Another important feature resides in the construction of the enginecasing which provides means by which a skirt on the compressor pistonspasses between the compressor inlet and outlet valves thus partiallyfilling the otherwise wasted space between the valves, at the inner endof the piston stroke thus reducing the volume of the head clearancespace, permitting the use of smaller compressors thus resulting in amore compact engine. This arrangement provides that the cooler outsideair entering the compressor through the inlet valves impinges directlyon the compressor outlet valves when entering the compressor cylinderthus providing some cooling of the outlet valves and also provides foreasy servicing of the engine by ensuring proper alignment of the variousparts when the engine is re-assembled. Again this arrangement providesthat a maximum number of valves can be contained on a minimum amount ofspace, thus increasing the efficiency of the engine.

These and other objects and features will become apparent from thefollowing detailed description taken in conjunction with the accompaningdrawings in which:

Fig. 1 is a part longitudinal mid vertical sectional, part elevationalview of a free piston gas generator embodying the invention and,

Fig. 2 is a fragmented, part exploded perspective view and partsectional View taken along the line 2-2 of Fig. 1.

The pistons are shown in their inward position. Arrows indicate thedirection of the flow of air and gas through the valves, ducts etc. Theengine is cylindrical in form and comprises a power cylinder 1,containing a pair of opposed power pistons 2, a fuel injector 3, inletports 4 and exhaust ports 5 which are opened and closed by the powerpiston. Compressor pistons 6 form an integralpart of the power pistonsand operate in compressor cylinders 7. The pistons are interconnected bya suitable synchronizing mechanism (not shown) to maintain the pistonsin phased relation. The outer portion of the compressor cylinders formbounce chambers 8 which provide cushions of air to store energy andreturn the pistons on the inward compression stroke. The inner part ofthe compressor cylinders form' -air compressor chambers 9 having inletvalves'lt) and outlet valves 11, through which air is delivered to theairbox 12 which surrounds the power cylinder.

tons to their outward stroke position (opposite to the position shown inthe idrawirigyQA'mass of compressed air is suddenly released into thebounce chambers which pushes the pistons inward, thus compressing air inthe power cylinder. At a suitable portion of the stroke,.fuel isinjected which ignites spontaneously driving the pise tons outward. Asthepistons move outward, the right hand piston first opens the exhaustports 5 releasing the gas from the power cylinderto the exhaust line 13which may be connected to a suitable A i (not illustrated). The lefthand piston then uncovers the inlet ports allowing a charge of air tosweep through the power cylinder and mix with the exhaust gas. On theoutward movementoffthe pistons air is compressed power producing turbinein the bounce chambers which serves to return the pistons on the nextinward stroke. During the outward stroke air is inducted into thecompressor chambers through the inlet valves 10. On the next inwardstroke, the air is compressed and delivered through the outlet valve 11into the airbox 12 where it is stored until the inlet ports 4 are openedby the power piston. The preceding engine operating principle is wellknown. The novelty of this engine resides in the following features.

To start the engine, the pistons are first moved to their outward strokeposition. A spool type valve 14 is connected to a source of highpressure air to port 15. In one position of the valve as shown, the highpressure air is directed through the valve into pipes 16. When the valvespool is shifted either manually or mechanically to the alternativeopposite position, the port 15 is closed off and the pipes 16 are openedto the atmosphere through port 17. The pipes 16 are connected to thestarting valves 18 at points 19 and 2%. A ball type check valve 21 isshown at point 20. This valve is held closed by the compression spring22 which exerts sufficient force to require say 10 pounds air pressureto open this valve. At point 19, the high pressure air enters thestarting valve and exerts a pressure against the piston valve 23 movingit in a direction to seat the valve face 24 against the valve seat 55against the compression of the spring 25 which requires say pounds airpressure to overcome the action of this spring. Air also enters thesmall cushion chamber 26 through the port 27.

When the piston valve closes, the air pressure now lifts the ball typecheck valve 21 and enters the air storage reservoir 28. Air also passesthrough the ports 29 into the reverse side or the piston valve 30. Sincethe area is smaller on this side of the valve 23, the valve will remainseated and'air will be held in the reservoir 28. A poppet type valve 31is attached to the end of the piston valve as shown. As long as airpressure is maintained in the starting valve, this poppet valve 31 isheld open.

When the spool valve 14 is quickly shifted, thus connecting the pipes 16to the atmosphere, there is a sudden drop in air pressure in the pipes1-6. The ball check valve 21 will then close, trapping the full airpressure in the reservoir28. Since the pressure in the pipes 16 has beenreleased, full pressure in the reservoir 28 is now exerted against thepiston valve 23 at 30 causing the valve to unseat and release the airfrom the reservoir 28 through the passage 32 into the bounce chamber 8.The very sudden relea'se'of the high pressure air into the bouncechamber tends to hammer the poppet valve 31 against its seat at 33. Toovercome this difiiculty, as thepiston valve 23 moves away from its seat55, it closes the port 27 thus trapping some air in the cushionchamber 26 which absorbs the shock yet allows free passage of air from thereservoir 25 into the bounce chamber. I

When the air is fully released from the reservoir, the spring25-closes'the poppet valve thus sealing the starting valve from thebounce chamber so that the volume of the bounce chamber is maintained atnormal.

valve 39 also connects to the airbox through ducts 37. It will be seenthat when the compressor pistons move inward an increasing pressure ofair in the airbox will cause the valve 39 to open and allow more air toenter the bounce chambers. Also, if the air pressure in the airboxdecreases, when the compressor pistons uncover the ports 34, the excessair will flow out of the bounce chambers through ports 34 and valves 35into the air-box. Thus it will be seen that the amount of air trapped inthe bounce chambers is maintained in direct proportion to the pressureprevailing in the airbox at various power levels, the mini-mum airpressure in the bounce chambers being approximately equal to theprevailing pressure in the airbox. This arrangement provides automaticcontrol of the outward stroke of the pistons at various power levels.

Referring in particular to Fig. 2 in conjunction with Fig. 1, the enginecasing comprises a central tubular section 4-1 which surrounds the powercylinder. Valve mounting rings 42 and 43 are bolted to the centresection 41 by screws 56. From Fig. 2, it will be seen that the innersurfaces of the valve mounting rings 42 and 43 have milled slots 44-which are spaced circumferentially around the inner surface of each ofthe valve mounting rings 42, and 43. Rectangular openings 45 areprovided through the wall of each ring in each slot 44.

The valve illustrated in Fig. 2 comprises a valve seat a valve reed erof thin flexible steel and a valve guard 48 having an arched inner faceagainst which the valve reed flexes when the valve is open. The valveseat and the valve guard may be faced with plastic material such as soldunder the trademark Teflon to reduce friction at the ends of the valvereed. The valves are assembled in the position shown and located in themilledslots 44 in both valve mounting rings. The compressor head 49 isclamped to the outlet valve mounting ring 43 by screws 5% which seat onthe clamp ring 51 which bears against the outlet valve mounting ring 43.The compressor cylinder is bolted to the inlet valve mounting ring '42by screws 52.

In the assembled position, it will be seen that the inlet and outletvalves associated with the valve mounting rings 42 and 43 respectivelyare radially disposed opposite to each other with a gap between theinlet and outlet valves at 53. A tapered skirt 54 0n the compressorpiston 6 enters the gap 53 on the inner pistonstroke and reduces thevolume of this annular gap to a minimum yet permits a passage adjacentthe inner face of the skirt so that air can pass out of the compressorchamber through the outlet valves. I The power cylinder 1 is accuratelysupported in the compressor cylinder heads 49. The composite boltedconstruction of the cylinder head 49 and the outlet valve mounting ring43 permits some expansion of the power cylinder without any unduedistortion of the engine casing When the starting air rushes into bothbounce chamhers 8 simultaneously, the pistons are driven inward and thecompressor pistons 6 uncover a series of radially disposed holes orports 34 through the wallsof the compress'or cylinders. These holes arelocated at a position in the stroke where the pistons have attainedsufiicient inward momentum to continue in their travel'to'cause ignition of the fuel in the power cylinders. The excess starting air passesout of the compressor cylinder through the Gil ports 34 whichcommunicate directly with air. check valves H 35 surrounding thecylinder permitting the air to pass out but preventing the passage ofair in the opposite direce tion; A collector ring 36 directs theexces'sair through ducts 37-intothe airbox at 3'8. Another air check valve 39located in the end wall 49' of the compressor cylinder permits air toflow into the bouncechaniber 8-but prevents the escape of air in theopposite direction. This due to heat. It will also be seen that thisconstruction permits easy fabrication and assembly of the engine casing.

While the compressor valves illustrated are of the well known feathervalve type, it will be understood that the efiicient venturi type inletvalves which form the invention of my co-pending US. patent applicationNo. 690,935 may be used instead.

,While I have described the preferred form of my invention, it willbeunderstood to those skilled in the art that various modifications andalterations may be made without departing from the spirit of myinvention or the scope of the appended claims.

N ha'tql claim as my invention is:

1. In a free piston gasgenerator having a power cylindercontainingopposed power pistons, compressor pistons operating incompressor cylinders at opposite ends of said. power cylinder, the outerportion of said compressor cylinders defining bounce chambers in whichair is adapted to be compressed on the outward piston stroke to storeenergy to return the pistons'on the inf ward stroke, the inner end ofsaid compressor cylinders defining air compressors adapted to deliverair into an airbox for scavenging the power cylinder, a piston strokecontrol comprising openings through the side wall of each of saidcompressor cylinders, one-way check valves opposite said openings topermit escape of air out of the respective bounce chamber into theairbox only when the respective compressor piston has moved past saidopenings on the inward stroke, said respective compressor piston beingadapted to close said openings on the outward stroke to trap air in saidrespective bounce chamber, another opening within said respective bouncechamber having one-way check valves opening into said respective bouncechamber to permit air from the airbox to flow into said bounce chamberso that the minimum pressure of air in the bounce chambers is maintainedapproximately the same as the prevailing pressure in the airbox.

2. In a free piston gas generator, a power cylinder, a compressorcylinder at each end of said power cylinder, a pair of opposing pistonseach having a portion disposed to operate in said power cylinder and acompressor piston portion disposed to operate in said compressorcylinder, the inner portion of said compressor cylinders forming aircompressors having inlet and delivery valves, said delivery valvesopening into a scavenging air receiver surrounding said power cylinder,the outer portion of said compressors forming bounce chambers in whichair is compressed on the outward stroke of the compressor pistonsstoring energy to return the pistons on the inward stroke, control meansto adjust the amount of air compressed in said bounce chambers inaccordance with the power output, said control means comprising, incombination, a duct from said scavenging air receiver opening into eachof said bounce chambers at a point beyond the stroke of said compressorpistons, at least one valve in said duct permitting the outward flow ofair from said scavenging air receiver into said bounce chambers throughsaid duct, at least one port through the side wall of at least one ofsaid compressor cylinders, said port being located within the stroke ofsaid compressor piston, a second valve associated with said port so thatwhen said piston uncovers said port on the inward stroke, and places airin said bounce chamber in communication with said second valve, anyexcess air in said bounce chamber will open said second valve releasingsaid excess air into said scavenging air receiver, the air in saidbounce chambers being trapped in said chambers during that portion ofthe outward stroke of the compressor pistons when said port is closed tosaid bounce chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,795,927 Huber June 18, 1957 2,815,641 Ramsey Dec. 10, 1957 2,839,035Ramsey et al June 17, 1958 2,849,995 Lewis Sept. 2, 1958 2,897,803Kolthofi Aug. 4, 1959

